At the moment, Organovo is synonymous with bioprinting in the 3D printing world. The San Diego–based company will begin selling its printed, living liver tissue later this year to the pharmaceutical industry, which can use it to test new drugs.

But other institutions are hot on Organovo’s trail. Scientists at the Harvard Wyss Institute for Biologically Inspired Engineering announced Wednesday that they have successfully printed multiple types of cells and blood vessels, a combination that is necessary to create more complex tissue. They published their work in Advanced Materials (subscription required). A visual walkthrough of their method is available here.

“Tissue engineers have been waiting for a method like this,” Wyss Institute founding director Don Ingber said in a release. “The ability to form functional vascular networks in 3D tissues before they are implanted not only enables thicker tissues to be formed, it also raises the possibility of surgically connecting these networks” to the human body.

Our cells need oxygen and nutrients. Without them, they die. It’s been difficult for researchers to 3D print thick pieces of tissue because the cells on the interior are cut off from the resources they need.

The Harvard team solved the problem by incorporating blood vessels into a mix of living cells and extracellular matrix, which connects cells to form tissue. While most materials harden as they cool, the researchers used a material that melts as it cools to map out blood vessels within the cells and matrix. Then they cooled the finished structure. The cells stayed in place while the other material melted, allowing the researchers to suction it out.

The team then filled the newly formed tunnels with another form of cells: endothelial cells, which make up the lining of blood vessels and naturally arranged themselves to form them in the 3D printed structure. The resulting network can feed cells on the interior, keeping them alive.

Like Organovo, the Harvard team sees drug screening as an immediate application for printed tissue. They could also study tumor development and how wounds heal.